In recent years, there has been a rapid advancement in the development of portable and cordless electronic devices. With this development, the commercialization of lithium ion secondary batteries having a high voltage and a high energy density as the power sources for driving these electronic devices is growing. The positive electrode for lithium ion secondary batteries usually uses, as the active material, a composite lithium oxide having a high oxidation-reduction potential, namely, a composite lithium oxide containing a transition metal such as lithium cobalt oxide, lithium nickel oxide or lithium manganese oxide. The negative electrode uses, as the active material, a material capable of absorbing and desorbing lithium ions. Examples of the material include alloy materials and carbonaceous materials. The electrolyte is usually a non-aqueous electrolyte prepared by dissolving a lithium salt such as LiClO4 or LiPF6 in an organic solvent. Between the positive electrode and the negative electrode is placed a sheet-like separator for electronically insulating the electrodes from each other. The separator is usually a microporous film made of polyolefin resin material.
The lithium ion secondary battery has a problem that, when it is repeatedly charged and discharged, its battery capacity gradually decreases. One possible cause for this is that both the positive and negative electrodes expand during charge and contract during discharge. Due to the repeated expansion and contraction, the electrolyte is expelled from the electrode group comprising the positive electrode and the negative electrode, which reduces the reaction area, eventually reducing the battery capacity. Particularly, the negative electrode expands and contracts to a greater extent, and therefore the retention of electrolyte by the negative electrode greatly affects the charge/discharge cycle characteristics.
When the positive electrode uses, as the active material, a composite lithium oxide having an oxidation-reduction potential of 4 V level relative to that of a lithium metal, the lithium ion secondary battery has a problem that the capacity decreases significantly when the battery is fully charged and then kept at a high temperature for a long period of time. Presumably, this is due to the side reaction which involves the decomposition of the electrolyte and the oxidation degradation of the separator which is in contact with the positive electrode.
In order to improve the safety of the lithium ion secondary battery, there is proposed to form a porous electron-insulating film having a thickness of 0.1 to 200 μm and comprising an inorganic oxide filler and a binder on the electrode surface. The porous electron-insulating film is intended to prevent the active material from separating from the electrode current collector during the production thereof, and the separated active material from being attached to the electrode surface again. It is expected to prevent internal short-circuiting and to help improve the production yield (see Japanese Laid-Open Patent Publication No. Hei 7-220759, for example).
Further, in order to prevent the oxidation degradation of the separator, there is proposed to use a multilayer film separator comprising a plurality of layers in which the layer to be in contact with a positive electrode contains polypropylene resin (Japanese Laid-Open Patent Publication No. 2001-273880).